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Technical Brief

Computational and experimental fatigue analysis of contoured spinal rods

[+] Author and Article Information
Agnese Piovesan

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
agnese.piovesan@mail.polimi.it

Francesca Berti

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
francesca.berti@polimi.it

Tomaso Villa

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
tomaso.villa@polimi.it

Giancarlo Pennati

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
giancarlo.pennati@polimi.it

Luigi La Barbera

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
luigi.labarbera@polimi.it

1Corresponding author.

ASME doi:10.1115/1.4042767 History: Received October 05, 2018; Revised January 29, 2019

Abstract

Posterior fixation with contoured rods is an estabilished methodology for the treatment of spinal deformities. Both uniform industrial preforming and intraoperative contouring introduce tensile and compressive plastic deformations, respectively, at the concave and at the convex sides of the rod. The purpose of this study is to develop a validated numerical framework capable of predicting how the fatigue behavior of contoured spinal rods is affected by residual stresses when loaded in lordotic and kyphotic configurations. Established finite-element models describing static contouring were implemented as preliminary simulation steps and were followed by subsequent cyclical loading steps. The equivalent Sines stress distribution predicted in each configuration was compared to that in straight rods and related to the corresponding experimental number of cycles to failure. In the straight configuration, the maximum equivalent stress (441 MPa) exceeds the limit curve, as confirmed by experimental rod breakage after 10^5 loading cycles. The stresses further increased in the lordotic configuration, where failure was reached at at 10^4 cycles. The maximum equivalent stress was below the limit curve for the kyphotic configuration (640 MPa), for which a run-out of 106 cycles was reached. Microscopy inspection confirmed agreement between numerical predictions and experimental fatigue crack location. The contouring technique (uniform contouring or French Bender) was not related to statistically significant differences. Our study demonstrates the key role of residual stresses in altering the mean stress component, superposing to tensile cyclic load, potentially explaining the higher failure rate of lordotic rods compared to kyphotic ones.

Copyright (c) 2019 by ASME
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